Usually, ultraviolet absorbents and/or ultraviolet scattering agents (zinc oxide, titanium oxide, etc.) are added to sunscreen cosmetics in order to block ultraviolet irradiation to the skin to achieve a high SPF (Sun Protection Factor) (refer to Patent Documents 1-3, for example). Examples of types of such sunscreen compositions include the oil-in-water emulsified type and the water-in-oil emulsified type.
The oil-in-water type as shown in Patent Documents 1-2 is preferably used to obtain refreshing and dewy, fresh texture. However, oil-in-water emulsified compositions cannot contain a large quantity of the oil component, which is to be the inner phase, due to the stability issue of the emulsified system.
Also, since stickiness arises when a large quantity of the oil component is blended in, it is not a common practice to blend in a large quantity of the oil component because the texture is of particular importance in cosmetics. Therefore, it is difficult to blend a large quantity of an oil soluble ultraviolet absorbent into an oil-in-water emulsified compositions while maintaining the characteristic refreshing, dewy, fresh and water like texture; as a result, it cannot have a high ultraviolet protection ability compared with other emulsified type sunscreen compositions.
Therefore, sometimes a water soluble ultraviolet absorbent, in addition to said oil soluble ultraviolet absorbent, is added so that the blend ratio of the oil soluble stays low and therefore a high ultraviolet protection ability is obtained while the dewy, fresh texture characteristic to oil-in-water emulsified compositions is maintained (see Patent Document 4, for example). However, when a water soluble ultraviolet absorbent is blended in, salt (neutralization salt) is usually blended in to maintain the system stability. This elevates the salt concentration in the emulsified composition and cancels out the electrostatic repulsion between the emulsified particles, which in some cases drastically worsens the emulsification stability.
In a water-in-oil emulsified sunscreen composition (see Patent Document 5, for example), the oil phase is the continuous phase (outer phase). Therefore, compared with oil-in-water sunscreen compositions, a larger quantity of the oil component and/or the oil soluble ultraviolet absorbent can be blended in to obtain a high ultraviolet protection ability. When it is used it leaves on the skin surface an oil film with a low moisture permeability that protects the skin from drying for a long time; furthermore, it doesn't re-emulsify easily when exposed to water in situations including bathing, washing/cleaning, and perspiration; for these reasons, it is used as an antiperspirant/water resistant base agent in sunscreen compositions.
However, a conventional water-in-oil emulsified sunscreen composition must dissolve a large quantity of a highly polar ultraviolet absorbent in the oil phase, resulting in separation and aggregation over a period of time; hence there is the problem of poor long term stability.
Also, in terms of texture as an external preparation, compared with an oil-in-water type sunscreen cosmetic, it has problems such as stickiness, oiliness, and poor spreadability.
On the other hand, whereas an emulsified composition is the non-equilibrium system, in which water is dispersed in oil, or oil is dispersed in water, as emulsified particles, microemulsion is an equilibrium isotropic low viscosity solution, in which water is solubilized in oil or oil is solubilized in water. There are three types of microemulsion, i.e. the micelle aqueous solution phase, in which oil is solubilized in water, the reverse micelle oil solution, in which water is solubilized in oil, and the bicontinuous microemulsion.
In the micelle aqueous solution phase, water forms a continuous state and sphere-like or rod-like aggregates, with the lipophilic groups of the surfactant facing inward, are dispersed. The size of the aggregate is up to about 100 nm; the external appearance is optically isotropic and transparent or bluish semi-transparent. When a micro aqueous solution is used as a sunscreen composition, it is generally not possible to solubilize a large quantity of oil components and therefore the blend ratio of the oil soluble ultraviolet absorbent cannot be raised, which makes it impossible to obtain a high ultraviolet protection ability.
The reverse micelle oil solution phase is a dispersion of spherical or rod-like aggregates having the hydrophilic groups of the surfactant facing inward. When a micro oil solution phase is used as a sunscreen composition, it is not possible to solubilize a large quantity of water and therefore stickiness and the oily sensation are pronounced, which makes it impossible to obtain a satisfactory texture during use.
On the other hand, the bicontinuous microemulsion phase is generally formed under conditions where hydrophilicity and hydrophobicity are in balance; because the interfacial tension becomes the minimum, the number of surfactant aggregates increases and form aggregates infinitely, and as a result the soluble quantity of water and oil drastically increases and thus a solubilized system having a specific structure of continuous channels of water and oil is formed.
It is said that searching for a combination of chemical compounds that meets the formation condition of a bicontinuous microemulsion phase is very difficult and the range of formation conditions is very narrow. Therefore, although it is often observed as a three phase state of oil phase/bicontinuous microemulsion phase/water phase in which it coexists with non-solubilized excess water or oil (Non-Patent Document 1, for example), there are not many reports of the bicontinuous microemulsion phase observed as one state. For example, it has been reported that the bicontinuous microemulsion phase can be obtained by either using polyoxyethylene lauryl ether (4E0) for the surfactant and isohexadecane for the oil component (Non-Patent Document 2), or using a didecyl methyl ammonium salt for the surfactant and dodecane for the oil component (Non-Patent Document 3), or using soy phospholipid for the surfactant, propanol for the detergency builder, and triglyceride for the oil component (Non-Patent Document 4).
Also, it has been reported that a bicontinuous microemulsion phase can be obtained by either using polyethylene glycol monolaurate (12E0) for the surfactant, lauryl alcohol and ethanol for the detergency builder, and liquid isoparaffin for the oil component (Patent Document 6), or using POE (8) glyceryl isostearate for the surfactant and cetyl octanoate liquid paraffin for the oil component (Patent Document 7), or using imidazolium betaine and POE lauryl ether sulfate for the surfactant, and a mono glycerol mono fatty acid ester and/or monoalkyl mono glyceryl ether for the oil component (Patent Document 8).
The bicontinuous microemulsion phase assumes a unique structure in which both water and oil are continuous and therefore, compared with the micelle phase, it can contain more oil components and/or or oil-based ingredients. Also, compared with the reverse micelle phase, it can contain more water and water-based ingredient. There are many reports of it as a cleaning agent that is highly capable of cleaning both oil-based stains and water soluble stains and rinses well (for example, see Patent Documents 6-10) wherein its ability to mix well with both water and oil and its low interfacial tension were utilized. However, currently, industrial utilization outside of cleaning purposes is marginal; no utilization is known for sunscreen compositions such as sunscreen cosmetics.